The electronics industry uses low dielectric constant and flexible materials in a variety of applications, including the fabrication of many devices, such as printed circuit boards and flexible circuits. Polypropylene and fluoropolymers, such as polytetrafluoroethylene (PTFE) exemplify some of the polymers that are used because of their low dielectric constant and low modulus. These material properties can be enhanced when the polymers are highly porous. The problems with these materials have been that material properties are often compromised when the porous material is attached to another material in the fabrication of the printed circuit board, flexible circuit, or other device.
In order to attach the porous polymer to another material, the polymer is either fused or bonded with an adhesive. The fusing process often involves high temperatures and pressures (PTFE requires 700F and 1000 psi) and is not practical in most applications. Use of an adhesive provides for ease of processing, but the properties of the adhesive are not as desirable as those of the polymer. Therefore the adhesive degrades the performance of the porous polymer. Attempts at minimizing the effect of the adhesive have centered around coating adhesive only on the surface to be bonded. Though this does work, the adhesive bond strength is often limited.
In addition, higher resin contents have also been used to provide good flow and adhesion around circuit traces on the surface being bonded. Though resin does flow to fill in the gaps, the porous polymer does not flow around the circuit traces. Lowering the amount of resin allows the porous polymer to flow around the traces, but with the existing technique of surface coating the resin, adhesion is compromised.
Other attempts to provide flexible printed circuit board materials include using porous polytetrafluoroethylene as a base sheet containing cured bismaleimide-triazine resin. Composites containing 40% and 60% adhesive by weight are formed from the bismaleimide-triazine adhesive and offer low dielectric and flexible properties to the composite material. In preparing this resin-containing polytetrafluoroethylene composite, bismaleimide-triazine resin is caused to permeate the porous node fibril texture of the expanded polytetrafluoroethylene material and is cured. As a result, the resin effectively enters the microstructure of the minute fibers and engages and binds itself with the minute fibers that give rise to an integral sheet-like material. The bismaleimide-triazine resin binds to the fibers and nodes in the interior as well as on the outer surfaces of the expanded polytetrafluoroethylene matrix material. However, there is no recognition that adhesive quantity is critical or that surface located adhesive is detrimental.
Other flexible polymeric composites for use in the electronics industry involve impregnating other resins into sintered expanded polytetrafluoroethylene. The resins include epoxy, polyamide, polyester, acrylic, triazine and bismaleimide/triazine resins. As a result of the sintering, the porous polytetrafluoroethylene has less flexibility. Here too there is no recognition that adhesive quantity and location are significant.
Although adhesive containing polymeric substrates have been prepared, almost no attention was placed on the particular mechanism used to add the adhesive to the polymeric matdx. In general, the adhesive was applied in an indiscriminate manner whereby the adhesive would not only be located within a portion of the composite matrix but would also flow over and cover exteriorly located fibril and node portions. For example, the adhesive was applied as a layered coating on one surface of the polymeric composite. As a result, the outer surface of the composite was essentially and substantially completely covered by an adhesive.
Crack-failure that occurs with time and use is propagated within a continuous layer of adhesive at the bond line that results from surface coating and/or high quantities of adhesives. These and other problems have been overcome by the present invention that provides a "discontinuous" layer of adhesive that in an uncompressed state remains within the void volume of the uncompressed polymeric matrix and where the coating of exterior surfaces of the polymeric infra-structure is substantially, if not totally, eliminated.